The present invention relates to methods for recovering hydrocarbons trapped in hydrate formations, and in particular to methods for the recovery of natural gas.
Hydrate formations are an example from a class of chemical compounds known as clathrates. A clathrate is a form of compound in which one component is enclosed within the structure of another. In a hydrate formation, hydrocarbon (such as natural gas) is trapped in molecular cage-like structures formed by frozen water. Although the water molecules and the hydrocarbon molecules do not interact chemically, the hydrocarbon molecules are completely surrounded by the water molecules. The structure of the hydrate must therefore be disassociated in order to free the hydrocarbon. A cubic meter of natural gas hydrate contains typically 160 cubic meters of natural gas.
Hydrate formations can be found in vast quantities in the earth's crust, principally in seabed sediments and in permafrost regions of the world. Conventional natural gas accumulations are thought to underlie many of these hydrate formations, the hydrate formations possibly having been formed when rising natural gas met subterranean water deposits, which then froze form hydrates and enclose the gas.
It is estimated that subterranean hydrate formations, either sub-sea or on land contain over twice the quantity of natural gas, as all the existing provable reserves of conventional accumulations of natural gas. Accordingly, there is a huge potential source of energy if the trapped natural gas can be recovered from these hydrate formations.
Although sub-sea hydrate formations can be located relatively easily by methods such as seismic profiling of the seabed, it has proved difficult or at least very expensive to extract the gas from hydrate formations. Various methods have been developed to address this problem, which employ heat, solution, and reduction of pressure or a combination of the three processes.
Heat can be used to disassociate natural gas from hydrate formations by the introduction of heated solutions into the hydrate formation. Such methods are often somewhat impractical however, due to the heat loss associated with transferring a heat-transfer in order to heat a formation which is located hundreds of metres under the permafrost, or thousands of metres under sea level. U.S. Pat. No. 4,424,866 discloses a method of recovering natural gas from gas hydrate formations by pumping a hot supersaturated solution of calcium chloride or calcium bromide under pressure into the hydrate formation so as to fracture the formation hydrostatically, dissolve the solid hydrate and release the gas. In an alternate heating method disclosed in U.S. Pat. No. 5,713,416, an acidic liquid and a basic liquid are combined and react exothermically to form a hot salt solution, which is injected into a gas hydrate formation in order to decompose the hydrate and release the gas. Both of these methods are uneconomical.
An alternative solution, which is discussed in U.S. Pat. No. 4,007,787 involves the introduction into the gas hydrate formation of freezing point depressants such as methanol, in order to disassociate the hydrate and free the natural gas. However, this method is also expensive, due to the cost of the freezing point depressant. It is also inefficient, since the depressants suggested do not lower the freezing point of the hydrate by an extent sufficient to liberate sufficient natural gas.
U.S. Pat. No. 4,007,787 disclose a pressure reduction method in which the hydrate is disassociated by reducing the ambient pressure. The advantage of this method is that the structure of the hydrate can be broken down without the need to increase the temperature. However, depressurisation methods are also considerably more expensive than conventional gas production methods.
GB-A-2250761 discloses the use of aqueous polysaccharide compositions in well-drilling and oil and gas recovery operations. The compositions can comprise ionic salts. The presence of the alcohol in the aqueous solution allows control of the transition temperature, without substantial variation in composition density. This reference does not address the problem of the recovery of hydrocarbons from hydrate formations.
WO-A-9726311 discloses the use of various mixed salt systems for use as various fluids concerned with drilling operations, such as hydraulic fracturing. This reference is not concerned with the particular problems which arise in the recovery of hydrocarbons from hydrate deposits.
U.S. Pat. No. 1,866,560 is concerned with a method for dehydrating gases, using solutions of calcium chloride. Again, it does not disclose any particular problems which arise in the extraction of hydrocarbons from hydrate formations.
U.S. Pat. No. 4,979,965 is also concerned with a method of dehumidifying gas, in which a salt solution is employed that contains salt crystals, so that the concentration of the salt will not be substantially reduced during absorption This reference is not concerned with hydrocarbon recovery from hydrate deposits.
WO-A-9818542 is concerned with dehydrating natural gases, using a dehydrating composition comprising a glycol and a dissolved salt. The reference is not concerned with hydrocarbon recovery from hydrate deposits.
Even if gaseous hydrocarbon can be extracted from the hydrate by one of the above processes using heat, solution or depressurisation, further problems can arise in transporting the liberated gas back to the ship or rig for refinement and storage. The difficulty is that the liberated gas will be wet, i.e. it will carry with it a proportion of water and water vapour from the hydrate formation, and, at sub-sea temperatures and pressures, the water vapour is likely to freeze, thereby re-forming gas hydrates, which can block the pipeline.